For example, a crawler belt for the undercarriage of a bulldozer is constituted by track shoes, track links, coupling pins, track bushings and seals. Each track shoe is fastened to laterally paired track links with bolts and nuts (see FIG. 1). Each track link has (a) a wheel tread which comes in rolling contact with a track roller, (b) a shoe mounting surface to which a track shoe is attached, (c) a pin hole for receiving a coupling pin inserted therein, (d) a bushing hole for receiving a track bushing inserted therein, (e) a seal counter bore to which a seal is fit, (f) shoe bolt holes for receiving track shoe clamping bolts inserted therein, and (g) shoe nuts bearing surface on which track shoe clamping nuts sit.
The above track link is conventionally produced by a production method comprised of such steps as shown in FIG. 5. Specifically, a round (square) bar made of high strength steel such as SMn based steel, SCM based steel or SNCM based steel is first prepared (Step 1). Then, this material is formed by hot forging into a link material which is substantially similar, in form, to a finished product (Step 2). Thereafter, the link material is entirely quenched by a quenching medium such as water or oil (Step 3). Subsequently, this quenched link material is tempered at a high temperature of about 400° C. to 650° C. (Step 4), thereby obtaining a high-strength, tempered, martensitic (sorbite) metal structure. In this way, the hardness of the link material is made to be HRC25 to HRC 40 to ensure fatigue strength. Then, the link material is machined at the parts to be formed into a wheel tread, shoe mounting surface, pin hole, bushing hole, seal counter bore, shoe bolt holes and shoe nut bearing surfaces, so that they have their respective specified sizes (Step 5). Of these machined parts, the wheel tread part is susceptible to wear caused by earth and sand because high surface pressure would be imposed on the wheel tread part with earth and sand trapped between the wheel tread part and the track roller. To withstand the wear caused by earth and sand, the wheel tread part undergoes induction quenching and tempering (Steps 6, 7) to form a hardened layer having a thickness of about 5 to 15 mm and a hardness of about HRC45 to HRC55. Then, finish machining is applied to the machined parts, thereby finishing a track link (Step 8).
A track link usually loses its function and ends its service life when the wear of the wheel tread proceeds so that the flange of the track roller interferes with the track link or when the wheel tread becomes so thin that it is not strong enough to bear the weight of the vehicle any longer. It is therefore necessary to improve the wear life of the wheel tread as much as possible. To this end, it is effective to increase the depth of the hardened layer of the wheel tread.
However, the surface hardening method such as ordinary induction quenching and tempering process has the disadvantage that the hardened layer to be formed with the method is relatively shallow (about 15 mm), no matter how good the hardenability of the used material. If the heated surface region is deepened with the intention of obtaining a deeper hardened layer, heating time becomes longer with the result that the temperature of the wheel tread part becomes too high, leading to an increase in crystal grain size and consequently a decrease in the toughness of the wheel tread part.
Further, if induction heating is again applied to the link material which has once undergone entire quenching and tempering treatment in Steps 3 and 4, the link material will have the temperature distribution shown in FIG. 6 and the part a which is insufficiently heated will be brought into a condition similar to the case where high temperature tempering is done. As a result, a layer (i.e., the so-called soft zone) having hardness lower than the hardness already attained by thermal refining is formed as indicated by IQT (Induction Quenching—Tempering) product line in the hardness distribution comparative graph of FIG. 4, so that the strength of the link material becomes insufficient and, in consequence, peeling of the hardened layer occurs.
The invention is directed to overcoming the above problems and a primary object of the invention is therefore to provide a track link production method capable of providing markedly improved wear life with good cost performance and a track link produced by this method.